Hi oris,
Sending it in to Dan is a good idea as he will get it in top shape. Do you know if the previous owner was "inside" the unit? We all know what DIY'ers can do to products.
I have sold a few of these preamps and nobody, to my knowledge, has ever mentioned this problem. I have had mine for a while and never experienced this or any other sonic problems.
Have no fear, you will soon experience the full potential this preamp is capable of and will probably never look back.
Keep us posted.